I found this great cartoon of the main cloud types:
It seems there are 3 main layers of cloud types: low clouds include stratus, cumulus and stratocumulus; middle layer clouds include altostratus and altocumulus and high level clouds include the wispy cirrus, cirrocumulus and cumulonimbus. There are other types too but these are the main types. Cumulus have "vertical displacement" and can therefore occur at any level and are the big bubbly clouds you tend to see associated with thunder storms.
Looking at this cartoon I think the majority of clouds I've drawn have been the low level types, mainly cumulus.
Possibly some altocumulus above (bottom right). Rest are probably cumulus.
These two above look like typical cumulus clouds.
I experimented using pastels on black paper. I think it's worked quite well. The pastels are good at creating the fluffy edges of the cumulus clouds. The dark paper shows through and creates a great effect and the white highlights on the clouds are more dramatic on the dark paper. Other colours of paper would be interesting to experiment with. If there was a lot of blue sky, an orange paper would be great to make the blue sky really vibrant. Similarly if it was a sunset and the clouds were coloured yellow or orange, a purple or blue paper would work quite well.
|A4 pastels on black paper|